1. Field of the Invention
The present invention relates to a pneumatic tire with sipes being formed on blocks, and particularly to a pneumatic tire in which grounding properties in a medium term of tire wear and thereafter has been secured for achieving improvements in tire performances.
2. Description of the Prior Art
While pneumatic tires are usually formed with circumferential grooves and transverse grooves on treads thereof, sipes of small width have been additionally engraved for improving drainage. Moreover, studless tires that aim for running on snowy and icy roads are formed with more sipes for exhibiting edge effects of the sipes and thus to improve braking, traction and cornering performances.
The rigidity of the blocks is generally degraded by forming such sipes. When increasing the number of sipes or the density thereof, the rigidity of the blocks may be excessively degraded. As a result, the blocks excessively collapse to decrease the ground contact area, thereby degrading the tire performance.
For preventing such excess degradations in the rigidity of the blocks, shapes of the sipes have been variously devised. As illustrated in FIG. 3, wave-shaped sipes 31 is formed on the blocks in a pneumatic tire as disclosed in Japanese Patent Application Laid-open No. 2002-321509. Crest portions 21 and trough portions 22 of the wave-shaped sipes 31 are arranged to extend not in a straight-linear manner but in a zigzag-like manner while extending from a tread surface 17 to a sipe bottom portion 18. As a result, while along the crest portions 21 and trough portions 22 blocks collapse in an aligned manner, the differences in aligning directions result in higher rigidity of the blocks. Consequently, excessive collapsing of the blocks is restricted so that edge effects of the sipes 31 are exhibited in a favorable manner.
However, according to the pneumatic tire of above mentioned invention, despite the fact that the height of the blocks in the medium term of wear and thereafter will decrease while the rigidity of the blocks is increased, the zigzags of the crest portions 21 and the trough portions 22 of the sipes 31 still extend up to the sipe bottom portion 18. As a result, no suitable collapsing of the blocks will occur due to extreme increases in rigidity of the blocks so that grounding properties will be worsened and edge effects will become insufficient.
Moreover, since the crest portions 21 and the trough portions 22 of the sipes extend obliquely from the tread surface 17 already in a new condition, acute angle portions 32 are formed at aperture portions of the sipes and uneven wear might progress therefrom.
It is accordingly an object of the present invention to suitably decrease the rigidity of the blocks, cause suitable collapsing of the blocks, to sufficiently exhibit edge effects of the sipes and to improve braking performances in the medium term of wear and thereafter.
For achieving the above subject, the inventors have arranged, upon intensive examinations, the present invention is a pneumatic tire with blocks being formed on a tread and with sipes being formed on the blocks,
wherein the sipes are classified, in a depth direction from a tread surface up to a sipe bottom, into a surface portion, an intermediate portion and a bottom portion, wherein the surface portion and the bottom portion open in a wave-shaped form with crest portion and trough portions of the wave-shaped form extending in directions vertical to the tread, and wherein the intermediate portion opens in a wave-shaped form with crest portions and trough portions of the wave-shaped form extending in a zigzag-like manner.
The present invention is a pneumatic tire with blocks being formed on a tread and with sipes being formed on the blocks,
wherein the sipes are classified, in a depth direction from a tread surface up to a sipe bottom, into a surface portion, an intermediate portion and a bottom portion, wherein the surface portion opens in a wave-shaped form with crest portions and trough portions of the wave-shaped form extending in directions vertical to the tread, wherein the intermediate portion opens in a wave-shaped form with crest portions and trough portions of the wave-shaped forms extending in a zigzag-like manner, and wherein the bottom portion opens in a straight-linear form.
The present invention is a pneumatic tire with blocks being formed on a tread and with sipes being formed on the blocks,
wherein the sipes are classified, in a depth direction from a tread surface up to a bottom of the sipe, into a surface portion, an intermediate portion and a bottom portion, wherein the surface portion opens in a straight-linear form, wherein the intermediate portion opens in a wave-shaped form with crest portions and trough portions of the wave-shaped form extending in a zigzag-like manner, and wherein the bottom portion opens in a wave-shaped form with crest portions and trough portions of the wave-shaped form extending in directions vertical to the tread.
The present invention is a pneumatic tire with blocks being formed on a tread and with sipes being formed on the blocks,
wherein the sipes are classified, in a depth direction from a tread surface up to a bottom of the sipe, into a surface portion, an intermediate portion and a bottom portion, wherein the surface portion and the bottom portion open in a straight-linear form, and wherein the intermediate portion opens in a wave-shaped form with crest portions and trough portions of the wave-shaped form extending in a zigzag-like manner.
When the blocks collapse, the crest portions and trough portions at an intermediate portion of the sipes will move into opposite directions since they extend in a zigzag-like manner. Due to the opposing directions of alignment, the rigidity of the blocks will be increased and excess collapsing of the blocks will be restricted. As a result, it is possible to secure a ground contact area for the blocks and to favorably exhibit edge effects of the sipes.
Accompanying the progress of wear of the tread, only the bottom portions of the sipes will remain so that the rigidity of the blocks becomes high while collapsing of the blocks will become insufficient. However, since the crest portions and the trough portions of the bottom portions extend in directions vertical to the tread, the rigidity of the blocks will be decreased so that excess collapsing will occur. As a result, it is possible to improve the grounding properties, to improve the edge effects, and to improve the tire performances such as braking performances. Similarly, when employing straight sipes which bottom portions open in a straight-linear form, the rigidity of the blocks will be decreased so that excess collapsing will occur to thus improve the edge effects.
Moreover, pullout resistance of sipe blade at the time of demolding from a mold will become small so that demolding becomes easy to thus prevent damages of the sipes.
By arranging the sipes as straight sipes that open in a straight-linear form or as wave-shaped sipes which crest portions and trough portions of the wave-shaped form extend in directions vertical to the tread are employed at the surface portions, no acute angle portions will be formed during the initial term of wear in which the rigidity of the blocks is low so that occurrence of uneven wear is restricted.
The present invention is the pneumatic tire further arranged in that where a depth of the sipes is D, a depth D1 from the tread surface to a boundary between the intermediate portion and the bottom portion is 40 to 90% of D, and a depth D2 from the tread surface to a boundary between the surface portion and the intermediate portion is 10 to 40% of D.
By setting D1 and D2 to be within specified ranges, it is possible to prevent uneven wear, to restrict excess collapsing of the blocks, and to exhibit sufficient edge effects during the initial term of wear. In the medium term of wear and thereafter, suitable collapsing of the blocks is made to occur to thus exhibit sufficient edge effects.
According to the present invention, by arranging the sipes as straight sipes which bottom portions open in a straight-linear form or as wave-shaped sipes which crest portions and trough portions of the wave-shaped forms extend in directions vertical to the tread, suitable collapsing of the blocks is made to occur to thus exhibit sufficient edge effects and to improve braking performances also in the medium term of wear and thereafter.